Recently, with the increased use of digital communication systems with a high peak-to-average power ratio (PAR) of signals such as WCDMA, WiBro, WiMAX, LTE, etc., there is a need to improve the linearity and efficiency of a power amplifier that is responsible for signal amplification in the corresponding system. Moreover, with the development of digital pre-distortion (PDP) linearity technologies along with the demand for miniaturization of the system, the use of a Doherty amplifier which is suitable for applying to the DPD linearity technologies and can reduce the power consumption and size has increased.
At present, the Doherty amplifier is fabricated using an FR-4 substrate having a dielectric constant of 4.8 or a Teflon substrate having a dielectric constant of 3.5. However, with the substrate having such low dielectric constants, it is difficult to reduce the size of the Doherty amplifier.
In general, an input of the Doherty amplifier comprises a distributor for distributing signals and a delay line for a phase delay of 90 degrees, and an output of the Doherty amplifier comprises matching circuits for output matching of the power amplifier and a 90-degree delay line for coupling outputs of a carrier amplifier and a peaking amplifier. Impedance varies in a region where the outputs of the carrier amplifier and the peaking amplifier are combined, and thus the Doherty amplifier further comprises an impedance conversion circuit for converting the impedance into the impedance of the output side. In order to obtain a desired impedance by the impedance conversion circuit, tuning of the impedance conversion circuit is required. In the case of a high power transistor, the tuning is performed by changing a pattern of a line or attaching a capacitor. However, the tuning in this manner reduces the efficiency of the operation and cannot achieve uniform quality, which are very problematic.
Meanwhile, a package comprising a metal such as Cu/Mo/Cu or CuW is used in the carrier amplifier and the peaking amplifier to fabricate a high power Doherty amplifier, and the use of this package increases the cost. Moreover, a structure in which transistors that constitute the carrier amplifier and the peaking amplifier are attached to the metal package basically occupies a large area, which becomes an obstacle to the miniaturization, and a fixing method using a screw also hinders the miniaturization.